If you have to ask, two more cogs in the back won't make you faster. Long term, after most of a season or three of conditioning, that might change. But as [MENTION=302603]bikemig[/MENTION] notes, the high and the low gears are similar, so you'll climb at similar speeds on both in the low gears, and you'll probably find you can't push as fast as the high gears allow. In between, you might find you accelerate quicker by downshifting a gear (like revving a car engine when you start to pass), but it takes a while to be able to turn the cranks much faster than you do at an easy pace.